Protein-protein interactions control practically all biological processes. Ability to manipulate these interactions is crucial for finding cures for the vast majority of human diseases. Conventional high-throughput screens for small molecule blockers of protein-protein interactions produce a disappointingly small number of lead compounds. In addition, screening procedures and subsequent structure optimization are laborious, lengthy and expensive. Meanwhile, specific folds inherent at the interfaces of the interacting proteins have been successfully mimicked for inhibition of the target interactions. Protein fragments involved in interaction can be used for inhibition of these interactions. However, peptides corresponding to fragments of protein primary structures tend to have little or no defined conformation, which results in low efficacy and poor stability in circulation. Consequently, the major effort in mimicking the interface is directed towards making the mimicking peptide or peptidomimetic as rigid as feasible. Strategies have been developed that allow simulation of reverse turns, beta-sheets and alpha-helixes. Cyclization is the most frequently used approach to affect stabilization of both beta-turns and alpha-helices. However, for inhibitors of intracellular protein-protein contacts, there remains the problem of cell permeability. Hydrocarbon-stapled alpha-helix peptidomimetics have demonstrated improved cell penetration, but this method is applicable only to helical fragments of proteins. We have recently found that membrane tethering stabilizes the structure of a peptide and converts it into a potent cell-permeable inhibitor of the corresponding protein. The approach allows for straightforward generation of potent and selective inhibitors of the target proteins. Utilizing the approach, we have developed selective inhibitors of STAT1, STAT3, STAT5 transcription factors, Hedgehog pathway, insulin-like growth factor 1 receptor, Jak1 and Jak2 kinases, interleukin 10 and interferon gamma signaling and Ras oncogenes. Inhibitors of STAT1, STAT3 and RAS oncogenes have been tested in mouse models of cancer and have demonstrated remarkable anti-tumor activity. Lipopeptides represent a new type of potential therapeutics with a wide range of applications. Lipopeptide inhibitors are also powerful chemical biology tools. Utilization of selective inhibitors of Ras oncoproteins allowed for uncovering of two previously unknown mechanisms of regulation of the proteins that are responsible for the growth of over 30% of all human tumors. Inhibitors of STAT3 N-terminal domain were instrumental in discovering that this transcription factor is involved in activation of transcription of apoptosis-inhibiting proteins in breast and prostate tumors, but not normal cells. The studies have identified STAT3 N-terminal domain as a promising drug target.